The right-wing Center for Immigrant Studies has a very interesting 2007 study here. Some excerpts:
"The vast majority of working-age illegals work. In fact, we estimate that 92 percent of illegal-alien households have at least one person working. This compares to 73 percent of native-headed households. "
"The primary reason for the high rates of immigrant poverty, lack of health insurance, and welfare use is their low education levels, not their legal status or an unwillingness to work."
"While immigrants overall are not more entrepreneurial than natives, immigrants from such countries as Korea, Iran, Italy, and Vietnam [and Poland] are significantly more likely than natives to be self-employed."